chore(docs): add multiple keys example to README (#503)

Author: bshaffer

README.md

@@ -202,6 +202,44 @@ $decoded = JWT::decode($jwt, new Key($publicKey, 'EdDSA'));
 echo "Decode:\n" . print_r((array) $decoded, true) . "\n";
 ````
 
+Example with multiple keys
+--------------------------
+```php
+use Firebase\JWT\JWT;
+use Firebase\JWT\Key;
+
+// Example RSA keys from previous example
+// $privateKey1 = '...';
+// $publicKey1 = '...';
+
+// Example EdDSA keys from previous example
+// $privateKey2 = '...';
+// $publicKey2 = '...';
+
+$payload = [
+    'iss' => 'example.org',
+    'aud' => 'example.com',
+    'iat' => 1356999524,
+    'nbf' => 1357000000
+];
+
+$jwt1 = JWT::encode($payload, $privateKey1, 'RS256', 'kid1');
+$jwt2 = JWT::encode($payload, $privateKey2, 'EdDSA', 'kid2');
+echo "Encode 1:\n" . print_r($jwt1, true) . "\n";
+echo "Encode 2:\n" . print_r($jwt2, true) . "\n";
+
+$keys = [
+    'kid1' => new Key($publicKey1, 'RS256'),
+    'kid2' => new Key($publicKey2, 'EdDSA'),
+];
+
+$decoded1 = JWT::decode($jwt1, $keys);
+$decoded2 = JWT::decode($jwt2, $keys);
+
+echo "Decode 1:\n" . print_r((array) $decoded1, true) . "\n";
+echo "Decode 2:\n" . print_r((array) $decoded2, true) . "\n";
+```
+
 Using JWKs
 ----------